The present invention relates generally to a method of operation and apparatus for noise attenuation of positive displacement compressors, and more particularly, to a method of operation and apparatus for noise attenuation of screw compressors that decreases the audible noise generated by one or more of the screw compressors by varying the operating speed of each screw compressor about its respective central frequency.
Heating and cooling systems typically maintain temperature control in a structure by circulating a fluid within coiled tubes such that passing another fluid over the tubes effects a transfer of thermal energy between the two fluids. A primary component in such a system is a positive displacement compressor, which receives a cool, low pressure gas and by virtue of a compression device, exhausts a hot, high pressure gas. One type of positive displacement compressor is a screw compressor, which generally includes two cylindrical rotors mounted on separate shafts inside a hollow, double-barreled casing. The side-walls of the compressor casing typically form two parallel, overlapping cylinders which house the rotors side-by-side, with their shafts parallel to one another. Screw compressor rotors typically have helically extending lobes and grooves on their outer surfaces forming a large thread on the circumference of the rotor. During operation, the threads of the rotors mesh together, with the lobes on one rotor meshing with the corresponding grooves on the other rotor to form a series of gaps between the rotors. These gaps form a continuous compression chamber that communicates with the compressor inlet opening, or “port,” at one end of the casing and continuously reduces in volume as the rotors turn and compress the gas toward a discharge port at the opposite end of the casing.
These rotors rotate at high rates of speed, and multiple sets of rotors, or multiple compressors, may be configured to work together to further increase the amount of gas that can be circulated in the system, thereby increasing the operating capacity of a system. While the rotors provide a continuous pumping action, each set of rotors produces pressure pulses as the pressurized fluid is discharged at the discharge port. These pressure pulses are generated by the compressor at increments of the operating speed of the driven screw, which is typically about 5 or 6 times the driven or operating RPM. These discharge pressure pulsations act as significant sources of audible sound within the system.
To eliminate or minimize the undesirable sound, noise attenuation devices or systems can be used. One example of a noise attenuation system is a dissipative or absorptive muffler system typically located at the discharge of the compressors. The use of muffler systems to attenuate sound can be expensive, depending upon the frequencies that must be attenuated by the muffler system. Typically, the lower the frequency of the sound to be attenuated, the greater the cost and size of the muffler system.
What is needed is an effective, low cost, efficient and easily implemented method or apparatus for compressor rotor noise attenuation.